This invention relates to a process for producing fine spherical powders by a microatomization process in which the starting material in powder form is melted and simultaneously accelerated to high velocities using a high temperature source such as a plasma jet and thereafter the material is impacted against a substrate which is heated to and maintained at a temperature above the melting point of the powder material.
Atomization is a commercial technique for producing metal powders. Atomization is achieved by directing a high pressure gas or fluid at a stream of molten metal. Resulting powders can be spherical or irregular in shape. Though the throughput rates are high the yield of fine powders (&lt;10 micrometer diameter) is low, typically in the 5-20% range.
Metal powders of various materials, metal, alloys, ceramics, and glasses can be reduced in size by milling. The resulting milled powders are irregular in shape. Contamination of the product can result from the grinding media and the liquid. These irregular powders can be spheroidized using a high temperature source (such as a DC plasma jet), if so desired. This two step process is subject to contamination problems. Melting of the milled particles using a high temperature source such as a DC plasma jet can also cause evaporation of fine powders. Unmelted particles can be included in the classified product.
A method for making fine metal powders, less than about 20 micrometers in size is described by Cheney et al in U.S. Pat. Nos. 4,502,885, 4,592,781, 4,613,371, 4,687,510, and 4,731,517 and by Johnson et al in 4,781,741. The processes relate to entraining powder material in a carrier gas and injecting into a high temperature source such as a DC plasma jet. The molten droplets are fragmented on impacting a substrate and are then resolidified in flight. As taught in the above mentioned patents, the substrate is cold, that is, at temperatures below the melting point of the powder material, and preferably chilled. They suggest also using a substrate that is moving, preferably rotating, to continuously change the substrate area exposed to the impacting particles and plasma gases thus keeping the substrate temperatures low. Use of auxiliary fluids to keep the substrate cold is also suggested. Examples of fine powders made using the process include metals and alloys, glasses and ceramics. The above mentioned process as described in the above mentioned patents will be subsequently referred to as "Cold Substrate Microatomization". Extended operation of this process can be prone to problems. Particles which are unmelted or which have resolidified in flight can deposit on the substrate. The gradual build-up of the uneven rough deposit on the substrate results in gradual decrease in the process efficiency due to decrease in the fragmentation of the impacting droplets.